Eliminate pests by examining with scope, in the soil around the roots as well. Fungus nats can cause a problem that apears this way. If you aliminate pests, then move on to over dose (what It looks like). After that you only have a couple factors that can cause this apperance root lock? could be... After eliminating all other factors run a good flush watch for rusty looking overflow out the bottom(If in soil) If in hydro check that your nutes are not shooting or droping ph.